1. Field of the Invention
The present invention relates to an optical element used in silver-film cameras, video cameras, still video cameras, copiers, and so on.
2. Related Background Art
There have been proposals made heretofore for a variety of optical systems utilizing reflecting surfaces of concave mirrors and convex mirrors, and U.S. Pat. No. 4,775,217 discloses the structure of an eyepiece in an observation optical system. This patent discloses the optical system that permits a viewer to observe a display image displayed on an information display medium and also to observe a scenery image of the outside as superimposed thereon.
Japanese Patent Application Laid-Open No. 2-297516 (corresponding to U.S. Pat. No. 5,453,877) also discloses the structure of the eyepiece in the observation optical system, and the whole optical system is made thinner by making use of total reflection of light between planes arranged in parallel.
These conventional examples disclose the technology of the so-called seethrough function by which the outside scenery image is superimposed on the display image in the information display medium by use of a semi-transparent film. The use of the semi-transparent film inevitably decreases the amount of transmitted light of the display image, but, in order to prevent it, the conventional examples employ the combination utilizing the total reflection to minimize loss of light amount at each reflecting surface. In many cases employing the total reflection at each reflecting surface, only planes are used as totally reflecting surfaces in order to simplify the structure. It is, however, more desirable to effect correction for aberration by each reflecting surface while optimizing the surface shape of each reflecting surface, rather than the structure in which the totally reflecting surfaces are only planes.
If an attempt is made to satisfy the total reflection condition for all beams incident to the reflecting surfaces, degrees of freedom of the surface shape will decrease, so as to bring about the disadvantage that efficient aberration correction cannot be made at the reflecting surfaces.
In order to solve these problems, the applicant of the basic application filed Japanese Patent Applications Laid-Open No. 8-292368 and Laid-Open No. 8-292371 to propose a reflection type optical system, wherein the whole optical system was compactified by use of an optical element in which reflecting surfaces of curved surfaces and planes were formed integrally, and wherein accuracy of placement of each reflecting surface was relaxed in the reflecting optical system.
In general, the video cameras etc. using solid state image sensors having a discrete pixel structure are designed to obtain an output image by optical space sampling of image information.
However, if on this occasion an object includes high spatial frequency components there will appear a false signal. It is thus necessary to optically limit the high frequency components of an image to below the Nyquist frequency. Optical filters having such function, i.e., low pass filters are classified into those making use of the birefringence effect of such a substance as quartz or the like and those making use of refraction and diffraction by surface shape like a phase grating.
Normally, in cases wherein a birefringent plate is used as a low pass filter, the birefringent plate is placed near the image plane. This imparts constraints on the design of an optical system and the design of mechanisms in the designing process which are demanded to compatify the system. Further, the interposition of the flat plate near the image plane will give rise to ghost and flare problems, so as to bring about the disadvantage of degrading the quality of image, and the like.
When a phase grating type low pass filter is used, the constraints on the design of mechanism can be relaxed by determining the grating pattern according to the distance from the image plane and a desired image separation amount. It is, however, necessary to pay attention to the point that the interposition of the transparent member should not degrade the optical performance including spherical aberration, curvature of field, and so on.
Further, when the optical system and the phase grating low pass filter are constructed as respective separate members, highly accurate alignment is required between them and improvement is necessary in the cost and production technology.
In view of the problems discussed above, an object of the present invention is to provide an optical element that can readily achieve the desired optical low pass filter effect while accomplishing compactification and simplification.
In order to accomplish the above object, an optical element according to the present invention is an optical element comprising an entrance refracting surface to which light is incident, a plurality of reflecting surfaces, and an exit refracting surface from which the light successively reflected by the reflecting surfaces emerges, in surfaces of a transparent body, wherein at least one of the two refracting surfaces and reflecting surfaces has the function of a low pass filter.